The present invention relates to electrical insulators such as electronic circuit boards and integrated circuit packages, and, in particular, to electrical discharges from such caused by ionizing radiation.
As the radiation hardness of electronic devices improves these devices will be able to survive increasingly higher levels of environmental irradiation. For photon or electron irradiations which produce surface doses exceeding 100 krad, the insulating materials become prone to prebreakdown discharges which can produce voltage pulses sufficient to damage or to electrically interrupt adjacent sensitive electronic devices. Insulation exposed to space radiation often becomes electrostatically charged so that the surface of the insulators are at a level of +10 to -10,000 volts relative to other elements of the spacecraft. When a spontaneous discharge initiates, it may propagate so that a large surface area is rapidly discharged. This rapid discharge of a large surface area results in a significant electrical pulse in adjacent or attached electrical circuits.
Nevertheless, it is known that damaging pulses do not occur when the insulator materials are coated with good "grounded" conductors. This solution can not be applied on many circuits where the possibility of a short circuit must be avoided. A plasma can be used to ground the surface and eliminate large pulses as effectively as a good conductor. The space plasma at many orbits is sufficient to do this naturally and is the main reason for large discharges being rare on spacecraft.
It is true that the higher energy radiations penetrate through the coatings and stop in the underlying insulation thereby producing high voltage in the insulator. Normally these deeply buried charges cannot produce large pulses in most grounded-surface situations. The large pulses occur when the free surfaces (adjacent to vacuum or to a gas) are at high relative potentials.
The discharges begin to occur when the surface of the insulator at issue receives an accumulated dose which approximates or exceeds 100 krads for typical electron space spectra. The discharges can occur only when the electric fields are sufficient to cause them; dose is not the cause of the pulses. However, the electric fields will not attain the critical strength until sufficient irradiation or secondary electrons have been stopped in the insulator which occurs typically at 100 krads.
The minimum dose must be provided within the dielectric relaxation time, which is typically of the order of days to years, in order for pulses to occur. Most spacecraft insulators will not accumulate sufficient charge within this time frame, only insulators close to a spacecraft surface will experience such charging.
However, dose rates and electrical charging rates near the surface of insulators such as circuit boards, insulated wires, connectors, antenna insulators, sensor insulators, device encapsulants and electrical feedthroughs have not normally been considered in light of these charging mechanisms. If surface dose were routinely considered, more devices would probably be found to have this problem. Ground based testing of electronics is often performed to a megarad or more (inside the device encapsulants) within a short time. Electrons, gamma rays and x-rays are all capable of producing these discharges.
A weakly conducting surface coating prevents significant radiation-induced charging of surfaces. Also, it will not cause short circuit current paths between electrically active components. Its level of conductivity is engineered in order to prevent radiation induced charging while avoiding current leakage paths.